Exemplary digital printers are known for example from DE 10 2010 015 985 A1, DE 10 2008 048 256 A1 or DE 10 2009 060 334 A1.
Two-sided printing, as for example in the case of two-sided duplex printing, in a printing system with subsequent combined fixing of the printed image on a first side and a second side opposite the first side, that is to say the front and back, of a recording medium is associated with significant problems when the front, which is printed first, is fed, after printing, directly to one or more printing units for printing on the back.
The greatest technical problem in this connection is that of finally feeding the print image, which is located on the front and which has not yet been fixed, to fixing without damaging the print image and with simultaneous printing on the back.
The unfixed print image on the recording medium, for example a printing substrate web, can be remobilised at any time under the effect of an electric field, as used for example in a printing unit of a duplex printing apparatus. In duplex printing, the printed image, for example a toner image, can on one hand be drawn onto the back of the recording medium by the electric field present between transfer roller and pressure roller (back roller). On the other hand, the already transferred print image of the front can be removed from the recording medium. In addition to damage to the print image, this also results in soiling of the pressure roller.
In general there is the option of intermediate fixing (thermally, by means of pressure, by means of solvent, infrared light, flashlight, etc.), but this is associated with other drawbacks. Furthermore, it is usually not economical to use two fixing stations. In the case of thermal fixing, the recording medium which is dehumidified after fixing causes problems in terms of printability. A high energy input is also necessary in this connection.
It should also be noted that thermal fixing stations are also relatively large, since the heat of the air and the energy input per unit area are low. When using mineral-oil-based ink systems, explosion prevention and observance of the applicable limit values for the exhaust air must also be observed.
Furthermore, the recording medium may shrink, for example in the case of paper and cardboard. In principle, this results in a relatively great deviation of the print image dimensions between front and back. The print image of the front is printed narrower owing to transverse shrinkage by 0.5 to 1% of the dimension transverse to the paper fibre and the back print is printed in the original width. Thus, without additional technical complexity, a congruent print from front to back is not possible, and this specifically is a requirement in letterpress printing. Remoistening and increasing the efficiency in the transfer can be solved only with great technical complexity. In addition, print fixing can lead to a change in the gloss.
DE 197 55 584 A1 for example thus describes melting the toner for fixing the intermediate image by means of a gas having a temperature between 150° C. and 400° C., and this can lead to the problems indicated above. Also, relatively large quantities of steam in the range of 160 I/sec are used for the method described therein.
Similar methods are described in U.S. Pat. No. 5,140,377 A, DE 2003 992 A1 and DE 103 01 587 A1.
DE 20 2004 020 953 U1 also deals with fixing a toner on a recording medium, and also explicitly addresses the problem of shrinkage of the recording medium. Since this problem is considered unavoidable in the solution described therein, a correction apparatus is proposed in order for the shrinkage not to have a negative effect on the ratio of the printing on the front and back of the recording medium.
An alternative fixing method is also known from DE 10 2004 009 987, in which the fixing takes place by means of a polymer film. However, this requires an additional application of material and also greater complexity in terms of machinery.